Astronomers searching for hints of leaky galaxies have picked up radiation from a star-forming galaxy in the nearby universe that behaves rather like some of the earliest stellar factories that gave a dark universe its first rays of light.
The galaxy known as J0921+4509, as described in the journal Science, is pushing out 50 solar masses' worth of new stars per year – about an order of magnitude greater than the Milky Way's rate. This galaxy could help scientists shed light on one of the earliest epochs after the universe's birth — a formative period known as reionization, which took place a few hundred million years after the birth of the universe.
During reionization, the first stars that were coalescing out of the soup of neutral gas filling the universe began to send radiation out into their surroundings, separating the electrons from protons in the neutral hydrogen around them. This process gave the first starlight to the dark universe and helped to determine its cosmic structure, said Sanchayeeta Borthakur, an astronomer at Johns Hopkins University in Baltimore.
"It has consequences for how [large-scale] structures formed, how galaxies got bigger and bigger … and what kind of dark matter makes up the universe," she said.
Scientists aren't sure exactly what caused reionization, but they knew that the process needed starlight that was much more energetic than visible light: extreme ultraviolet rays or higher, known as Lyman continuum radiation.
But even though these young stars in the universe were presumably putting out a whole lot of Lyman continuum radiation, it's unclear how extreme ultraviolet radiation could have escaped and affected the surrounding universe. That's because the same clouds of neutral gas that give birth to the stars also imprison their light. If that high-energy ultraviolet could not escape, then how did reionization happen?
"That has been the quest, the mystery for astronomers for the last couple of decades," Borthakur said, "to understand if and how the galaxies were able to let their ionizing photons escape so that they could go out and ionize the universe."
Researchers have found what appeared to be "leaky" galaxies, which appeared to be letting light escape the cloudy prisons. But these galaxies seem few and far between – high-energy ultraviolet is not easy to look for, and it often quickly gets used up as it ionizes the atoms it hits. The handful of other Lyman continuum emitting galaxies in the nearby universe are allowing something like 2% or 3% of that radiation to escape. That's about a tenth of what would have been needed to reionize the universe.
For this study, the researchers looked for these special galaxies in an indirect way; they first scanned for slightly lower-energy radiation to narrow the list of targets and then homed in on the shortlist by looking for Lyman continuum with NASA's Hubble Space Telescope.
To trigger reionization, it's estimated that a galaxy would need to be leaking some 20% of its Lyman continuum radiation. Lo and behold, J0921+4509 was leaking 21%.
How was this crucial radiation escaping? In some star-forming galaxies, the stars could be packed extremely close together – perhaps a billion solar masses across a mere 300 light years. All those newly forming stars stuffed into this tiny patch of real estate can generate such strong winds – perhaps around 2.2 million mph – that they blow holes through the wall of neutral gas, allowing light to escape.
Now, this galaxy is relatively nearby in the universe – about 3 billion light-years away. But the scientists say these galaxies offer a valuable analogue they can use to study aspects of reionization – since, for now, it's virtually impossible to study it in the very distant galaxies, which are closer to the birth of the universe (which is 13.8 billion years old). That may change with better telescopes on the way, Borthakur said.